Add file_path validator

Implement a new provider::validatefx::file_path validator.

Description:

• Validate cross-platform file paths.

Acceptance criteria:

1. Subtask: Implement file_path validator in internal/validators
2. Subtask: Add unit tests for file_path validator
3. Subtask: Add file_path function to internal/functions package
4. Subtask: Add unit tests for file_path function
5. Subtask: Register file_path in registry.go
6. Subtask: Add documentation in examples/functions/file_path/function.tf
7. Subtask: Update docs table using make docs command
8. Subtask: Add integration test in integration/main.tf